This invention generally relates to tie-down assemblies, and is specifically concerned with a tie-down assembly for securing a cask for transporting radioactive waste to a semi-tractor trailer.
Casks for transporting radioactive materials such as the waste products produced by nuclear power plant facilities are known in the prior art. The purpose of such casks is to ship radioactive wastes in as safe a manner as possible. Such casks may be used, for example, to ship high-level vitrified waste cannisters to a permanent waste isolation site, spent fuel rods to a reprocessing facility, or low-level wastes such as spent ion exchange resins used to purify the water used in steam generators. At the present time, relatively few of such transportation casks have been manufactured and used since most of the wastes generated by nuclear power plants are being stored at the reactor facilities themselves. However, the availability of such on-site storage space is steadily diminishing as more and more waste is loaded therein. Additionally, the United States Department of Energy (DOE) has been obligated, by way of the National, Waste Policy Act of 1983, to move the radioactive wastes from the on-site storage facilities of all nuclear power plants to a federally operated nuclear waste disposal site starting in 1998. Thus the need for such casks may be expect to grow.
Tie-down arrangements for securing such casks to conventional road vehicles, such as tractor-trailers, are also known in the art. Such arrangements generally include a plurality of lug assemblies which are welded to the cask walls. These lug assemblies provide a convenient means of attaching the cask to an arrangement of tensioned cables which are in turn connected to the corners of the trailer deck. In order to comply with federal regulations, these tie-down arrangements must meet, two conflicting criteria specifically set forth in 10 C.F.R. Sections 71.45b(1) and 71.45b(3). The first criteria is that the cask lugs used in the tie-down arrangement must be capable of sustaining vertical, longitudinal and transverse static forces equivalent to two, ten and five times the weight of the cask and its contents, respectively. The second criteria requires each tie-down lug which is a structural part of the cask to be designed so that failure of the cask lug under excessive load or accident conditions would not break or rupture the cask walls and thereby breach containment. In short, the cask lugs must be strong enough to withstand the G forces set forth in the first criteria, yet weak enough to yield or break off of the cask before accident-induced stresses which the lugs apply to the cask walls would cause the walls to rupture.
While tie-down arrangements have been designed which do comply with the specifications set forth in 10 C.F.R. Sections 71.45b(1) and 71.45b(3), applicants have observed that there is considerable room for improvement in these prior art designs in at least three major respects. First, the overall factor of safety of such prior art tie-down arrangements is commonly about 1.01 or less. Hence, if the design of the tie-down arrangement is mis-executed, or the construction of the arrangement is any way defective, these tie-down arrangements would not comply with federal regulations, thus enhancing the possibility that a cask may become ruptured under accident conditions. Secondly, none of the tie-down arrangements of which applicants are aware seem to be specifically designed to withstand the 5-10-2G regulatory criteria. Instead, these designs seem to be directed (along the horizontal plane, at least) toward resisting forces equally along the longitudinal and transverse axis of the trailer. Consequently, most prior art tie-down arrangements anchor the cask to the outer edges of the trailer deck in an attempt to minimize the moment forces expected to be applied to the cas walls during an accident condition. However, such an anchoring configuration requires the outer edge of the trailer deck to be specially reinforced if the tie-down assembly is to have any chance of successfully resisting the high forces anticipated.
Clearly, what is needed is a tie-down assembly capable of meeting the aforementioned criteria by a substantially larger factor of safety, and which obviates the need for specially reinforcing the trailer upon which it is carried. Moreover, it would be desirable if such a tie-down assembly were designed to specifically resist the hypothetical 5-10-2G forces spelled out in the pertinent regulations. Such a tie-down assembly should further be compatible with existing cask designs, and easy to install therewith. Finally, the tie-down assembly should be easy to manufacture, and formed from relatively inexpensive and readily available materials.